We're not talking about sales tax here, but income tax. The general rule is that whenever you sell an item for more than you paid for it, the profit is taxable income. The rule usually would apply to people who buy and sell stuff for a living, and it would apply if you sold for example a baseball card collection for more than you paid for the cards when you collected them. It also applies to personal electronic devices but of course, most of the time you sell used items, you get less than you paid so the money is not taxable.
